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Cytosine () (
symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical meaning (linguistics), m ...
C or Cyt) is one of the four
nucleobases 230px, Pyrimidine nucleobases are simple ring molecules. Nucleobases, also known as ''nitrogenous bases'' or often simply ''bases'', are nitrogen-containing biological compounds that form nucleosides, which, in turn, are components of nucleoti ...
found in
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
and
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Re ...

RNA
, along with
adenine Adenine (A, Ade) is a nucleobase 230px, Pyrimidine nucleobases are simple ring molecules. Nucleobases, also known as ''nitrogenous bases'' or often simply ''bases'', are nitrogen-containing biological compounds that form nucleosides Nucleos ...

adenine
,
guanine Guanine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical me ...

guanine
, and
thymine Thymine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical m ...

thymine
(
uracil Uracil () ( U or Ura) is one of the four s in the that are represented by the letters A, G, C and U. The others are (A), (C), and (G). In RNA, uracil binds to via two . In , the uracil nucleobase is replaced by . Uracil is a form of . Ura ...

uracil
in RNA). It is a
pyrimidine Pyrimidine is an aromatic In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts ...

pyrimidine
derivative, with a
heterocyclic 125px, Pyridine, a heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different chemical element, elements as members of its ring(s). Heterocyclic chemistry is the branch of organi ...
aromatic ring forms of benzene (top) combine to produce an average structure (bottom) In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: the ...
and two substituents attached (an
amine In organic chemistry, amines (, ) are compounds Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), ...

amine
group at position 4 and a keto group at position 2). The
nucleoside Nucleosides are s that can be thought of as s without a . A nucleoside consists simply of a (also termed a nitrogenous base) and a five-carbon sugar ( or 2'-deoxyribose) whereas a nucleotide is composed of a nucleobase, a five-carbon sugar, and ...

nucleoside
of cytosine is
cytidine Cytidine (symbol A symbol is a mark, sign, or that indicates, signifies, or is understood as representing an , , or . Symbols allow people to go beyond what is n or seen by creating linkages between otherwise very different s and s. All ( ...

cytidine
. In Watson-Crick
base pair A base pair (bp) is a fundamental unit of double-stranded nucleic acids Nucleic acids are biopolymer Biopolymers are natural polymers produced by the cells of Organism, living organisms. Biopolymers consist of monomeric units that are Covalent_ ...
ing, it forms three
hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
s with
guanine Guanine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical me ...

guanine
.


History

Cytosine was discovered and named by
Albrecht Kossel Ludwig Karl Martin Leonhard Albrecht Kossel (; 16 September 1853 – 5 July 1927) was a German and pioneer in the study of . He was awarded the Nobel Prize for in 1910 for his work in determining the chemical composition of s, the genetic subst ...
and Albert Neumann in 1894 when it was hydrolyzed from calf
thymus The thymus is a specialized primary lymphoid organ of the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''or ...

thymus
tissues. A structure was proposed in 1903, and was synthesized (and thus confirmed) in the laboratory in the same year. In 1998, cytosine was used in an early demonstration of
quantum information processing Quantum information science is an interdisciplinary field that seeks to understand the analysis, processing, and transmission of information using quantum mechanics Quantum mechanics is a fundamental Scientific theory, theory in physics that p ...
when Oxford University researchers implemented the Deutsch-Jozsa algorithm on a two
qubit In quantum computing Quantum computing is a type of computation Computation is any type of calculation A calculation is a deliberate process that transforms one or more inputs into one or more results. The term is used in a variety of sens ...
nuclear magnetic resonance quantum computer (NMRQC). In March 2015, NASA scientists reported the formation of cytosine, along with uracil and thymine, from
pyrimidine Pyrimidine is an aromatic In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts ...

pyrimidine
under the space-like laboratory conditions, which is of interest because pyrimidine has been found in meteorites although its origin is unknown.


Chemical reactions

Cytosine can be found as part of DNA, as part of RNA, or as a part of a
nucleotide Nucleotides are organic molecules , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation, ...

nucleotide
. As
cytidine triphosphate Cytidine triphosphate is a pyrimidine Pyrimidine is an aromatic heterocyclic compound, heterocyclic organic compound similar to pyridine. One of the three diazines (six-membered heterocyclics with two nitrogen atoms in the ring), it has the ni ...
(CTP), it can act as a co-factor to enzymes, and can transfer a phosphate to convert
adenosine diphosphate Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of o ...

adenosine diphosphate
(ADP) to
adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properti ...

adenosine triphosphate
(ATP). In DNA and RNA, cytosine is paired with
guanine Guanine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical me ...

guanine
. However, it is inherently unstable, and can change into
uracil Uracil () ( U or Ura) is one of the four s in the that are represented by the letters A, G, C and U. The others are (A), (C), and (G). In RNA, uracil binds to via two . In , the uracil nucleobase is replaced by . Uracil is a form of . Ura ...

uracil
( spontaneous deamination). This can lead to a
point mutation A point mutation or substitution is a genetic mutation where a single nucleotide base is changed, inserted or deleted from a DNA or RNA sequence of an organism's genome. Point mutations have a variety of effects on the downstream protein product ...

point mutation
if not repaired by the
DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene mol ...

DNA repair
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
s such as uracil glycosylase, which cleaves a uracil in DNA. Cytosine can also be
methylated In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These t ...

methylated
into
5-methylcytosine
5-methylcytosine
by an enzyme called
DNA methyltransferase In biochemistry, the DNA methyltransferase (DNA MTase, DNMT) family of enzymes catalysis, catalyze the transfer of a methyl group to DNA. DNA methylation serves a wide variety of biological functions. All the known DNA methyltransferases use S- ...
or be methylated and
hydroxylated In chemistry, hydroxylation is can refer to: *(i) most commonly, hydroxylation describes a chemistry, chemical process that introduces a hydroxyl group (-OH) into an organic compound. *(ii) the ''degree of hydroxylation'' refers to the number of O ...
to make
5-hydroxymethylcytosine
5-hydroxymethylcytosine
. The difference in rates of deamination of cytosine and 5-methylcytosine (to uracil and
thymine Thymine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical m ...

thymine
) forms the basis of bisulfite sequencing.


Biological function

When found third in a
codon The genetic code is the set of rules used by living cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or reli ...

codon
of
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Re ...

RNA
, cytosine is synonymous with
uracil Uracil () ( U or Ura) is one of the four s in the that are represented by the letters A, G, C and U. The others are (A), (C), and (G). In RNA, uracil binds to via two . In , the uracil nucleobase is replaced by . Uracil is a form of . Ura ...

uracil
, as they are interchangeable as the third base. When found as the second base in a codon, the third is always interchangeable. For example, UCU, UCC, UCA and UCG are all
serine Serine (symbol Ser or S) is an α-amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, ...

serine
, regardless of the third base. Active enzymatic deamination of cytosine or 5-methylcytosine by the
APOBEC image:Apobec.J.Steinfeld.D.png, 300px, upExample of a member of the APOBEC family, APOBEC-2. A cytidine deaminase from ''Homo sapiens''.; ; rendered usinPyMOL APOBEC ("apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like") is a fam ...
family of cytosine deaminases could have both beneficial and detrimental implications on various cellular processes as well as on organismal evolution. The implications of deamination on 5-hydroxymethylcytosine, on the other hand, remains less understood.


Theoretical aspects

Cytosine has not been found in meteorites, which suggests the first strands of RNA and DNA had to look elsewhere to obtain this building block. Cytosine likely formed within some meteorite parent bodies, however did not persist within these bodies due to an effective
deamination Deamination is the removal of an amino group from a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically n ...
reaction into
uracil Uracil () ( U or Ura) is one of the four s in the that are represented by the letters A, G, C and U. The others are (A), (C), and (G). In RNA, uracil binds to via two . In , the uracil nucleobase is replaced by . Uracil is a form of . Ura ...

uracil
.


References


External links and citations


Cytosine MS Spectrum
* * {{Authority control Nucleobases Amines Pyrimidones